Coupler Insertion Loss

Application Note

Equation for the loss of the main line due to power loss through coupling factor.  This does not account for material losses.  The table at the bottom of this page has pre-calculated loss for most coupling values.

Recall dB = 10∙log(Pout/Pin).

  • Consider a 6 dB coupler. 
  • If a 1 watt signal is applied to the INPUT, then 0.25 watts appears at the COUPLED port. 
  • This comes from -6dB = 10∙log(1/4). 
  • Consequently, 0.75 watts must therefore be present at the OUTPUT port, in a lossless coupler, because total energy out = total energy in. 
  • In this example, the measured loss from INPUT to COUPLED is -6dB = 10∙log(1/4).
  • The measured loss from INPUT to OUTPUT is therefore -1.25dB = 10∙log(3/4).
Further Discussion

Please remember, this is a mathematical model based on an ideal situation.  This demonstration does not factor in additional loss due to material properties and imperfect matching. 

To understand the maximum specified insertion loss for a particular coupler model, always consult the datasheet.

Click here to view the directional coupler models listing.

Coupling (dB)Main Line Insertion Loss (dB)
3.03.021
3.52.570
4.02.205
4.51.903
5.01.651
5.51.438
6.01.256
6.51.101
7.00.967
7.50.850
8.00.749
8.50.661
9.00.584
9.50.517
10.00.458
11.00.359
12.00.283
13.00.223
14.00.176
15.00.140
16.00.110
17.00.088
18.00.069
19.00.055
20.00.044
21.00.035
22.00.027
23.00.022
24.00.017
25.00.014
26.00.011
27.00.009
28.00.007
29.00.005
30.00.004
35.00.001
40.00.000
45.00.000
50.00.000
  • Provided for convenience only.  Not responsible for errors.  Use at your risk.